1. Field of the Invention
The present invention relates to an insulating adhesive and an anisotropically electroconductive adhesive for use in, for example, establishing electric connections between circuit boards. More particularly, it relates to an insulating adhesive and an anisotropically electroconductive adhesive curable at low temperatures.
2. Description of the Related Art
In recent years, as countermeasures against the miniaturization of portable remote terminals such as portable telephones, and a crack in a liquid crystal cell or glass, there has been extensively examined the practical utilization of a so-called “plastic film liquid crystal” obtained by replacing the glass of the liquid crystal cell with plastic.
Conventionally, the connection between a liquid crystal panel and a circuit component such as a TCP (Tape Carrier Package) has been accomplished by solder, an anisotropically electroconductive adhesive film, heat seal, or the like. However, a thermosetting anisotropically electroconductive adhesive film has been mainstream in response to the trend toward a smaller pitch and a narrower frame, and from the viewpoints of high adhesion and high reliability. Thus, the plastic film liquid crystal is also required to be connected by the anisotropically electroconductive adhesive film.
The anisotropically electroconductive adhesive film is originally designed under the assumption that the connection between glass and TCP, or the like is established by compression bonding connection at high temperatures and high pressures, for a short time. In a common case, the connection is established, for example, at a temperature of 170° C. for about 20 seconds. Even in the case for a low-temperature connection, it is established at a temperature of 150° C. for around 20 seconds.
However, the plastic film liquid crystal which is a material to be joined is manufactured by laminating an organic material of a protective layer or the like on mainly polyether sulphone or polycarbonate. Therefore, at the compression bonding temperatures thereof, the film may undergo thermal deformation which will not occur with glass, or cracks occur in ITO (Indium Tin Oxide) electrodes, thereby causing a display failure.
For these reasons, the anisotropically electroconductive adhesive film for connecting the plastic film liquid crystal is required to have the characteristics of establishing the connection at 140° C. or less, and under low pressures. However, in actuality, among the existing anisotropically electroconductive adhesive films, there is almost no one which is connectable at 140° C. or less, and even the one connectable at 140° C. or less has a low reliability.
Heretofore, the thermosetting anisotropically electroconductive adhesive films which can provide high reliability include various epoxy resins as main components, and there have been dominant the ones obtained by adding a so-called latent curing agent prepared by micro-capsulating or blocking an amine-based or imidazole-based curing agent, Lewis acid, or other curing agent thereto, and processing and forming the mixture into a film.
Alternatively, there are also other films obtained by adding various resins such as thermosetting elastomers and thermoplastic elastomers, thermosetting resins, thermoplastic resins, tackifiers, fillers, and coupling agents thereto for the purpose of improving various characteristics such as adhesion, moisture resistance, and stickiness.
However, for the conventional anisotropically electroconductive adhesive films using such curing agents, since a compression bonding temperature of 150° C. or more is required for breaking or melting of the microcapsules, or for dissociation of the blocking agents, the foregoing requirements cannot be satisfied. Whereas, for the curing agents connectable at 150° C. or less, since the usable time length of the anisotropically electroconductive adhesive film (film life) is short, it is difficult to use the film in actual manufacturing thereof.
On the other hand, as the anisotropically electroconductive adhesive films which do not contain epoxy resins as main components, there is proposed one of the radical polymerization type obtained by effecting polymerization with an unsaturated bond catalyst (Japanese Patent Laid-Open Publication No. Sho 61-276873) by the applicant of the present invention, and the like. However, there has not been yet developed any anisotropically electroconductive adhesive film which satisfies the requirements of low-temperature curability, high adhesion, high reliability, and the like.